If you're installing an Ethernet for the first time in your life, a few
words about the cabling may be in order here. Ethernet is very picky
about proper cabling. The cable must be terminated on both ends with a
50-Ohm resistor, and you must not have any branches (i.e. three cables
connected in a star-shape). If you are using a thin coax cable with
T-shaped BNC junctions, these junctions must be twisted on the board's
connector directly; you should not insert a cable segment.

If you connect to a thicknet installation, you have to attach your
host through a transceiver (sometimes called Ethernet Attachment
Unit). You can plug the transceiver into the 15-pin AUI port on your
board directly, but may also use a shielded cable.